Method for making an optical micromirror and micromirror or array of micromirrors obtained by said method

ABSTRACT

A method for manufacturing an optical micro-mirror including a fixed part and a moveable part, with a reflection device connected to the fixed part by an articulation mechanism. This method realizes a stack including a mechanical substrate, a first layer of thermal oxidation material, and at least one second layer of material for forming the moveable part, realizes the articulation mechanism, realizes the reflection device on the second layer, realizes the moveable part by etching of at least the second layer of material, and eliminates the thermal oxidation layer to liberate the moveable part. Such an optical micro-mirror may find possible applications to optical routing or image projection systems.

FIELD OF THE INVENTION

The invention relates to a method for manufacturing an opticalmicro-mirror and a micro-mirror or array of micro-mirrors obtained bythe method of the invention. These micro-mirrors are capable of beingelectrically controlled.

Micro-mirrors are used generally in systems implementing deflections oflight beams and in particular in optical routing systems or in imageprojection systems.

BACKGROUND OF THE INVENTION

Electrically controlled micro-mirrors (most often using electrostatic,electromagnetic, piezoelectric, or thermoelastic forces) capable ofgenerating digital or analog angular positions are known in theliterature. They generally use hinge configurations making it possible,according to the complexity of the technological steps employed, tooscillate around an axis (simple hinge) or around two axes (doublehinge) of rotation oriented most frequently orthogonally.

FIG. 1 a represents a view of such an electrostatically controlledmicro-mirror enabling rotation on 2 perpendicular axes, utilized inoptical routing systems. The fixed frame 2 of the micro-mirror and themovable parts 3 and 4 articulated, respectively, around hinges 5 and 6that enable the desired rotations about the two orthogonal axes are madeon the substrate 1. Each axis of rotation passes through a distincthinge. The moveable part 4 is covered with a high-reflectivity layer.

FIG. 1 b represents a highly diagrammatic cross-sectional view of thedifferent elements forming this type of micro-mirror (section along theaxis of the hinge 5). In addition, in this Figure the different controlelectrodes 7, 8, 9 and 10 of the micro-mirror are represented. Theopposing electrodes 7 and 8 make it possible to turn the moveable part 3about the axis 5, which the opposing electrodes 9 and 10 make itpossible to turn the moveable part about the axis 6.

The manufacturing steps comprise, starting with a mechanical substrate,a sequence of deposits and etchings of suitable material enabling therealization of the different elements of the micro-mirror ormicro-mirrors (control electrodes, moveable parts, hinges, etc.) andcomprise the use of one or a plurality of sacrificial layers, removal ofwhich makes it possible to liberate the moveable part(s).

There are many technological alternatives for obtaining such devices. Inthis respect, the references cited at the end of the description can beconsulted.

Although in the detail of the structures and the sequences oftechnological steps implemented use a wide diversity of approaches, thedevices developed today have the following points in common:

-   the materials used for producing the moveable part or parts of the    micro-mirrors are, in the majority of cases, amorphous or    polycrystalline (polycrystalline silicon, aluminum, various metals,    etc.) deposited using very classical techniques (vacuum evaporation,    cathodic sputtering, vapor phase deposition, CVD, etc.)-   the materials used for producing the sacrificial layers can be of    different types (silica, various organic materials, etc.) but are    always obtained by deposition techniques (CVD, rotary deposition,    cathodic sputtering, etc.) that generally do not afford very precise    control of the thicknesses utilized (typically several tens of    nanometers for micron thicknesses) but that have the advantage of    being very flexible to use.

The drawbacks of the prior art approaches are at several levels:

-   First of all, unsatisfactory precision in angular excursion    (typically between 10⁻¹ and 10⁻²) as the result of the use of    sacrificial layers produced by deposition techniques that do not    have very high degrees of thickness control.-   For certain system architectures, in particular those used for    optical routing purposes, all of these points are prejudicial and    none of the manufacturing methods proposed in the prior art makes it    possible to overcomes them correctly.-   Moreover, poor mechanical properties of the amorphous or    polycrystalline layers of the thin layers constituting the moveable    part(s) that translates inter alia into a greater fragility and    deformation after clearing, which perturbs the planeity of the    surface.

This point is particularly important in the case of high surfacemicro-mirrors (of the order of a square millimeter or a fraction of asquare millimeter) which must carry out an image function demandingexcellent quality of optical wave surface.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method for formingan optical micro-mirror as well as the optical micro-mirror or an arrayof micro-mirrors obtained according to the method of the invention andnot having the drawbacks of the prior art.

In particular, the micro-mirror obtained according to the method of theinvention has excellent angular excursion precision, while havingsatisfactory angular excursion. In addition, the moveable part of themicro-mirror obtained according to the method of the invention hasmechanical properties that result in obtaining excellent planeity. Themicro-mirror of the invention can also be a hinged micro-mirror (singleor double) as well as a pivoting micro-mirror; this latter type ofmicro-mirror is original and can just be obtained in virtue of themethod of the invention.

More precisely, the object of the invention is a method formanufacturing an optical micro-mirror comprising a fixed part, amoveable part connected to the fixed part by articulation means, themoveable part comprising in addition reflection means. This methodcomprises the following steps:

a) realization of a stack formed of a mechanical substrate, ansacrificial layer of a specific thickness of thermal oxidation materialcalled the first layer and an assembly for forming the moveable part andcomprising at least one layer of material called the second layer;

b) realization of the articulation means;

c) realization of the moveable part by etching of at least the secondlayer of material so as to obtain at least one pattern;

d) removal at least in part of the sacrificial layer so as to clear saidmoveable part that is then connected to the rest of the micro-mirrorcorresponding to the fixed part by means of the articulation means.

The steps of the method of the invention can be carried out in theaforesaid order or in a different order; moreover, in certainembodiments, certain steps can be included in other steps. According tothe invention, the substrate or the layers are not strictly formed froma single material; thus, the substrate can comprise a plurality oflayers and the layers can comprise a plurality of sub-layers.

According to the invention, the use of a layer of thermal oxidationmaterial makes it possible to have a layer whose thickness is extremelywell-controlled that serves as sacrificial layer. The value of theangular excursion of the moveable part can thus be very precise andreproducible; it can also of a significant level.

Advantageously, the sacrificial layer of thermal oxidation material hasa thickness of greater than or equal to 1 micron.

In the case of a silicon substrate, for example, the method of theinvention makes it possible to obtain layers of thermal silica with aprecision of the order of several nanometers on total thicknesses ofseveral microns or several tens of microns, approximately 10 timesbetter than those obtained in the prior art regarding thicknesses of thesacrificial layers; the gain in angular precision resulting therefrom isalso of the order of a factor of 10.

The thermal oxidation layer can be partially removed; it must be etchedat least in order to make it possible to clear the moveable part.

Preferably, the reflector means are realized on the second layer bydeposition of a monolayer or multiple layers of reflector material suchas metals like gold, silver, aluminum, for example, or dielectrics ofSiO₂/TiO₂ or SiO₂/HFO₂, for example; these materials are deposited, forexample, by cathodic sputtering or vapor evaporation on the secondlayer, generally after step b).

If the second layer has sufficient reflectivity for the intendedapplication, the reflector means are then realized on the second layerafter epitaxy.

Advantageously, the method comprises in addition a step of epitaxy ofthe second layer, the reflector means being then realized using thesecond layer itself.

Epitaxy of the second layer makes possible an increase in the thicknessof this layer with the best mechanical continuity possible and obtainingof a minimally deformable layer with high mechanical quality (namelymechanical strength) that conserves excellent planeity even after thestep d) clearing step.

According to a preferred embodiment of the invention, the second layeris a layer of monocrystalline material. The use of monocrystallinematerial for the moveable part makes it possible to obtain high surfaceplaneity on which the reflectivity layer is deposited.

According to a first embodiment of the invention, step a) comprises therealization of the thermal oxidation layer on the substrate, then thedeposition of the second layer on the thermal oxidation layer.

Deposition is defined according to the invention as any type ofdeposition including the transfer of a layer.

For step a) either the different steps can be carried out or a wafer ofsemiconductor on insulator such as SOI commonly known as “Silicon onInsulator”, which is commercially available, can be used. In the latterinstance, by way of example, the SOI substrates can be used to advantageusing a wafer of thermal silica (for example, the wafers sold under thetrade name “Unibon” by the SOITEC company).

According to a second embodiment of the invention, step a) comprises thetransfer onto the mechanical substrate of the second layer, thesubstrate and/or the second layer comprising on their surfaces to becoated a thermal oxidation layer that will form after the application ofthe first layer.

Advantageously, the transfer comprises a sealing step (of the substrateor of the oxide on the one hand and of the second layer or the oxidelayer on the other hand) by molecular adhesion. A sealing element couldalso be used for this sealing such as an adhesive, for example.

The second layer can be combined with an intermediate substrate by aliaison zone capable of enabling the removal of the intermediatesubstrate after application or, in certain instances, prior toapplication.

According to a first use of this application, said liaison zone is anembrittlement zone obtained by ion implantation (see especially U.S.Pat. No. 5,374,564-U.S. Pat. No. 6,020,252) and/or by the creation ofporosity in the second layer, removal of the intermediate substrate isdone along this embrittlement zone by an appropriate treatment such asthe application of mechanical force and/or the utilization of a thermaltreatment.

According to a second use of this transfer, this liaison zone is asacrificial layer that is subjected to chemical attack in order toenable removal of the intermediate substrate.

The transfer technique used in this second embodiment makes possible theuse of at least two wafers advantageously assembled using molecularadhesion techniques and also makes it possible to overcome thelimitations of angular excursion without sacrificing precision relativeto the thickness of the sacrificial layer(s) (which sets the conditionslargely for the precision regarding angular excursion).

It also makes it possible to have a greater freedom for the realizationof complex structures without sacrificing the fundamental advantagesoffered by the invention (mechanical quality of the moveable parts andprecision of the angular excursions).

The thermal oxide layer is carried out preferably by high-temperatureoxidation under dry atmosphere (between 800° C. and 1,100° C. underoxygen) or under humid atmosphere (between 800° C. and 1,100° C. underwater vapor) and at atmospheric pressure or under high pressure.

According to one advantageous method for realizing the articulationmeans of the invention, before step d), local etching of the layer(s)disposed on top of the substrate is done so as to form at least one viaand epitaxy is across each via, the epitaxial material in each viaforming all or part of an articulation element of the articulationmeans.

The articulation elements can be produced respectively in several parts,especially in the case of the second embodiment using the transfer ofthe second layer. Thus, the articulation means of the invention arerealized by:

-   local etching prior to application in such a fashion as to form at    least one first via in the layer or layers disposed on top of the    substrate and in such a fashion as to form at least one second via    in the layer or layers disposed on the second layer, opposite to the    substrate;-   epitaxy through the first via forming one part of an articulation    element and epitaxy in the second via forming another part of the    articulation element, these two parts being brought into opposition    during the transfer and forming an articulation element after the    transfer.

According to a first embodiment of the articulation means of amicro-mirror, a single articulation element is realized and disposedunder the moveable part in such a fashion as to form a pivot for saidpart, said pivot connecting the moveable part to the fixed part. Thepivot may or may not be centered under the moveable part, depending onthe applications.

According to a second embodiment of the articulation means of amicro-mirror, two articulation elements are realized and disposed oneither side of the moveable part in such a fashion as to form a hingeconnecting it to the fixed part.

Preferably, according to this second method, the articulation means arerealized by etching the second layer; this etching may be done at thesame time as that of creation of the moveable part. Naturally,hinge-type articulation means can also be realized as hereinbeforedescribed, by epitaxy across the vias.

According to a preferred embodiment of the invention the substrate iscomprised of silicon, the first layer is a thermal oxide of silicon, thesecond layer is monocrystalline silicon and the articulation means arecomprised of monocrystalline silicon.

Advantageously, the method of the invention uses an attenuation of thesecond layer for reducing the inertia of the moveable parts and allowsfunctioning of the micro-mirror at higher frequencies.

This attenuation of the second layer can be realized either by thecreation of an embrittlement zone at a depth in the second layer so thatthe remaining thickness, after removal of the surplus (the surplus canbe an intermediate substrate), corresponds to the thickness desired forthe second layer, either by using a chemical etching or reactive ionstep or mechanochemical polishing until obtaining the desired thicknessor even a combination of all of these techniques. If the attenuationstep results in excessively thin thicknesses of the second layer, thisthickness can be restored in an epitaxy step.

According to an advantageous embodiment of the invention making itpossible to have high angular excursion of the moveable part, at leastone cavity is made in the mechanical support opposite at least one zoneof one of the extremities of the moveable part by etching of thesubstrate according to a form and geometric dimensions that make itpossible to depart from the dimensional parameters of the micro-mirrorand total angular excursion Δθ along the axis or different axes ofrotation.

The cavity or cavities of the substrate are advantageously realized byanisotropic etching, for example by wet etching or by dry processes suchas ion etching or reactive ionic etching. Generally, the substratecomprises in the case of a pivoting micro-mirror a peripheral cavityopposite to a peripheral zone of the extremity of the moveable part.

According to one embodiment, the micro-mirror being electricallycontrolled, the method of the invention comprises a step for realizationof control means by the formation of opposing electrodes on themechanical substrate and on the moveable part.

Advantageously, if the substrate and the moveable part are at least inthe facing semiconductor parts, the electrodes are formed by ionicimplantation of dopants whether or not followed by suitable thermaldiffusion of the dopants.

The connection lines from the electrodes to a control electronics can berealized in different ways and especially also by ionic implantation ofdopants whether or not followed by suitable thermal diffusion of thedopants. These lines are realized advantageously on the face of thesubstrate opposite the moveable part, the electrodes of the moveablepart being connected to certain of these lines advantageously by meansof articulation means. In addition, connections can be provided at theends of these lines with a view of their connection to the controlelectronics.

According to another embodiment, the connection lines of the differentelectrodes are realized by plated-through holes through the substrate;the electrodes of the moveable part being connected to certain of saidplated-through holes advantageously by means of the articulation means;connectors can in addition be provided at the ends of these lines with aview of their connection to the control electronics.

The invention can also make use of electrical control means utilizingforces other than electro-static forces and, for example,electromagnetic forces or piezoelectrical forces or even thermoelasticforces. By way of example, control of the moveable parts by magneticforces (Laplace forces) requires the realization of adapted coils andmagnets for generating the necessary magnetic fields.

According to one particular embodiment of the invention, the moveablepart comprises at least two parts: a first part comprising thereflection means and at least one second part surrounding the firstpart, the articulation means connecting said second part to the fixedpart and intermediate articulation means connecting the first part ofthe moveable part to the second part.

The articulation means of a micro-mirror can comprise at least one hingeor a pivot. The intermediate articulation means comprise at least onehinge.

Said hinge is realized advantageously by etching of the second layeraccording to a suitable pattern.

According to the invention, the use of a pivot makes it possible for themoveable part to move in all directions around an axis of symmetrypassing through the pivot and perpendicular to the plane of thesubstrate.

When the articulation means and the intermediate articulation means areformed by hinges, in general one hinge composed of 2 elements isnecessary for articulating each part of the moveable part, the elementsof the hinge being disposed on either part of said moveable part. Eachhinge allows a displacement of the part with which it is associatedaround an axis passing through the elements of the hinge called thehinge axis and which is parallel to the plane of the substrate. In orderto increase the degrees of freedom of the moveable part, each hinge isdisposed in such a fashion that its axis describes a specific angle,generally 90° to the axis of the other hinge, in a plane parallel to thesubstrate.

The method of the invention is applicable as well to the realization ofan individual micro-mirror and to a array of micro-mirrors; thesemicro-mirrors being capable of being controlled independently of eachother.

The invention relates also to the micro-mirror obtained according to thehereinbefore described method as well as to an array of suchmicro-mirrors.

According to the invention, the term array includes the strip, which isa special case of an array, in which the elements are arranged along asingle axis.

BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics and advantages of the invention will be moreapparent in light of the following description. This description refersto exemplary embodiments, provided by way of illustration butnon-limiting. It refers in addition to the attached figures, wherein:

FIG. 1 a–b already described, represents a hinged micro-mirror of theprior art that utilizes amorphous or polycrystalline materials forrealizing the moveable part and the sacrificial layer;

FIGS. 2 a to 2 i diagrammatically represent in cross-section thedifferent steps of a first method for manufacturing a micro-mirroraccording to the invention;

FIGS. 3 a to 3 g diagrammatically represent in cross-section thedifferent steps of a second method for manufacturing the fixed part of amicro-mirror according to the invention;

FIGS. 4 a to 4 g diagrammatically represent in cross-section thedifferent steps of a second method for manufacturing the moveable partof a micro-mirror according to the invention;

FIGS. 5 a to 5 e diagrammatically represent in cross-section thedifferent steps making possible, after transfer of the structuresobtained in FIGS. 3 g and 4 g, realization of a micro-mirror accordingto this second mode;

FIGS. 6 a to 6 g diagrammatically represent in cross-section thedifferent steps of a third manufacturing method of the fixed part of amicro-mirror according to the invention;

FIGS. 7 a to 7 c diagrammatically represent in cross-section differentpositions of a moveable part connected to the fixed part by means of apivot;

FIGS. 8 a and 8 b, respectively, provide an overall perspective of anexample of pivot micro-mirror and an example of a simple hingedmicro-mirror according to the invention;

FIGS. 9 a to 9 c represent top views of different micro-mirrors of theinvention showing in particular different geometries of electrodesmaking possible rotations about one (FIG. 9 a), two (FIG. 9 b) or four(FIG. 9 c) axes of rotation.

DETAILED DESCRIPTION OF THE EMBODIMENTS

There are, of course, numerous alternatives making possible realizationof the micro-mirrors of the invention.

We shall describe only two methods for manufacturing a micro-mirrorknowing on the one hand that these methods make possible a collectiveformation of micro-mirrors and on the other hand that these methods arenon-limiting. In addition, for the sake of simplification of thedescription, the case of utilization of articulation means employing apivot has been chosen that has the advantage of enabling, using a singlemoveable part, rotations about a plurality of axes perpendicular to theaxis of the pivot by simple modification of the geometry of the controlelectrodes and, by way of example, silicon has been chosen for thesubstrate, the second layer and the articulation means. These examplesare, of course, non-limiting.

The first method is realized on a wafer while the second method isrealized on two separate wafers A and B then transferred.

The first embodiment of the micro-mirror of the invention that isimplemented on a wafer is illustrated in the different FIG. 2.

For this (see FIG. 2 a in cross-section) a SOI (silicon on insulator)wafer is created or a wafer of this type available commercially is used.

In order to create this type of wafer, a non-doped silicon substrate 21is used, onto which a dielectric layer 22 of thermal silica is grown. Asurface monocrystalline silicon layer 20 is then deposited using any ofthe known deposition methods and in particular those for transferring athin layer.

FIG. 2 b represents the realization of the electrodes of the electricalcontrol by the formation of different doped zones 24, 24′, and 23 in thesuperior part of the non-doped silicon substrate 21 and in themonocrystalline silicon surface layer 20. These zones are obtained byionic implantation of dopant atoms (generally boron or phosphorus) atdifferent energies according to the desired depth of localization,whether or not followed by thermal annealing. According to the desiredlocalization depths and the thickness of the dielectric layer 22, theimplantation energies will be typically between 20 and 300 keV and theimplanted doses between 10¹⁴ and 10¹⁶ cm⁻². By way of example, in thelayer 20 having a thickness W′ of typically between 0.1 micron and 0.6micron, the implantation energies for forming the zones 23 will be low(15 to 100 keV); whilst in the substrate 21 the implanted ions must passthrough the silica layer 22 having a thickness W and in part the siliconlayer 21, the implantation energies for forming the zones 24 and 24′will be higher (generally greater than 100 keV). For a single-patternmoveable part a single doped zone 23 can suffice.

FIG. 2 c represents the formation of the site 25 of the future pivot bylocal etching of the layers 20 and 22 in order to form a via preferablyabove an implanted zone 24′.

FIG. 2 d represents an epitaxy step. This step makes possible at oncerealization of the doped monocrystalline silicon pivot and increasingthe thickness of the surface silicon 20 in order to enhance themechanical rigidity of that which will form the moveable part of themicro-mirror.

Realization of the articulation means is advantageously done usingmonocrystalline silicon in order to make it possible to obtainmechanically solid articulation means.

During the epitaxy step, doping of the epitaxial material can bemodified and, for example, chosen to be higher at the start of theprocess (corresponding to the formation of the pivot 27 that isadvantageously electrically connected to an implanted zone of thesubstrate) than at the end of the process where it is only a matter ofincreasing the thickness of the layer 20 for forming the monocrystallinesilicon layer 26, whose thickness can attain several microns dependingon the desired specifications. The depression 28 which can appear inthis epitaxial layer results from the presence of the local etching 25.

FIG. 2 e represents a section of the device after the epitaxy step andattenuation, for example, by mechanochemical polishing necessary forclearing the depression 28 and obtaining a monocrystalline silicon layer26 of perfect planeity. Other attenuation techniques can, of course, beused and in particular those described in the U.S. Pat. No. 5,374,564 orU.S. Pat. No. 6,020,252.

FIG. 2 f represents the realization of the reflection means by theformation on the layer 26 of a high reflectivity mirror layer 29 formicro-mirror usable wavelengths, for example, by metallic or multilayerdielectric deposition.

FIG. 2 g represents the etching step of the future moveable part of themicro-mirror. This etching, whose geometry and dimensions depend on theexpected optical specifications and thus the intended applications (forexample, square sides or circle diameters of the order of several tensof microns to several millimeters), uses layers 29 and 26 and eventuallythe thermal silica layer 22.

This etching is done, for example, by any type of etching adapted to thematerials used (ionic etching, reactive etching and/or chemicaletching).

By way of example, for layers 29 of aluminum, 26 of silicon, thisetching is done through a mask (not shown) by a first reactive ionicattack, for example using chlorinated gases for attacking the aluminum,then by a second reactive ionic attack, for example using an SF₆ gas forattacking the silicon.

FIG. 2 h represents a cross-section of the component after removal ofthe sacrificial silica layer 22 at least under the moveable part of themicro-mirror and hence the clearing of this moveable part. Removal ofthe layer 22 is done, for example, for a silicon oxide layer by chemicalattack using fluorhydric acid or by reactive ionic attack usingfluorinated gases.

In the structure represented in FIG. 2 h, the amplitude Δθ of the totalangular excursion is determined by the height H of the pivot and thewidth L of the moveable part in its plane of rotation (sine Δθ=H/2L);the ends of the moveable part of the micro-mirror can then be situatedabutting the substrate plane. This configuration thus has the drawback,for a given pivot height H, of entirely linking the total angularexcursion Δθ and the dimension L of the moveable part in the plane ofrotation considered.

FIG. 2 i provides a means for averting this drawback by creatingcavities 19 in the support 21 whether crossing or not, whose insideborders are situated at a distance L′ from the axis of pivot less thanL/2 and the outside borders at a distance L″ greater than L/2.

The angular excursion Δθ defined by the relation tangent Δθ=H/L′ doesnot depend then on L′ and not on L.

This cavity can be easily realized using the posterior surface of thewafer, for example by chemical etching preferably as illustrated in FIG.2 i and consequently must cross the thickness of the silicon substrate.

The second embodiment of the invention that carries out the steps of themethod on two wafers A and B then which transfers these wafers isrepresented in FIGS. 3, 4, 5.

Preparation of the A Wafer

Using a mechanical support, for example an undoped silicon wafer 31(FIG. 3 a), the different electrodes 33, 33′ of the fixed part isrealized by ion implantation of dopants whether or not followed bythermal annealing (FIG. 3 b). FIG. 3 c represents a thermal oxidationstep of the substrate for forming a thermal oxide layer 32 of aperfectly controlled thickness and generally between 1 and 3 microns; inthe course of this step done generally at high temperature, there is adiffusion of the dopants from the implanted zones and an increase of thevolume occupied by these zones.

The steps represented in FIG. 3 b and FIG. 3 c can be reversed at thecost of augmentation of the implantation energies for realizing thedoped zones 33 and 33′ (the ions implanted prior then cross the thermalsilica layer).

FIG. 3 d represents the following step corresponding to the localetching 34 of the thermal silica layer 32 on top of the doped zone 33′for forming a via. Then, FIG. 3 e represents an epitaxy step of thesubstrate that makes it possible to grow doped monocrystalline siliconin the via 34. The part of the articulation element 35 thus formed is ofa thickness generally very slightly greater than the thick ness of thesilica layer 32; this part of the element will constitute one part ofthe future pivot. FIG. 3 f represents a mechanochemical polishing stepintended to smooth the surface of the wafer A and “erase” any excessthickness from the articulation element 35.

FIG. 3 g represents a cavity 36 etching step that makes it possible todepart form the dimensions of the moveable part and the maximal angularexcursion Δθ of said part. The dimensions (position relative to the axisof the future pivot, width and depth) of the openings 36 are determinedusing the dimensions of the moveable part and of the desired angularexcursion Δθ along the different axes of rotation.

Contrary to the case, wherein the method of the invention is realizedusing one wafer and wherein the cavities 19 must cross the substrate, inthis second embodiment, wherein the method is realized using two wafersthat are then transferred, the cavities 36 can have a thickness muchless that the thickness of the substrate 31. These cavities can be ofany shape.

Preparation of the B Wafer

FIG. 4 represent the different steps for manufacturing the B wafer.First of all, a substrate 41 (FIG. 4 a), for example made ofmonocrystalline silicon, is used in which an electrode 43 is formed, forexample by ionic implantation of dopants (FIG. 4 b), whether followed ornot by thermal annealing. Then, a thermal oxide layer 42 (FIG. 4 c) isformed in the same fashion as for the layer 32. This layer 42 is thenetched to form a via 44 (FIG. 4 d) that extends up to the electrode 43;this opening has dimensions very close to those of the opening 34 (FIG.3 d); an epitaxy step (FIG. 4 e) using monocrystalline silicon thenmakes it possible to form in the opening 44 another part of thearticulation element that is made of doped monocrystalline silicon 45. Amechanochemical polishing step (FIG. 4 f) allows, if necessary,obtaining a perfect smoothness of the surface of the B wafer.

The step illustrated in FIG. 4 g consists of creating a liaison zone 46in the wafer 41 such as an embrittlement zone created, for example, byion implantation. This zone delimits in the wafer a layer (hereinbeforecalled the second layer) of a thickness of typically between 0.1 and 2microns between the silica layer 42 and the rest of the wafer (which canbe an intermediate substrate). This embrittlement zone makes it possibleto separate the second layer from the rest of the wafer, either beforetransfer but more generally after transfer (see in particular the U.S.Pat. No. 5,374,564 and U.S. Pat. No. 6,020,252).

Assembly of the A and B Wafers

The first step represented in FIG. 5 a consists of assembling the twowafers A and B, oxidized face against oxidized face. During thisassembly, the positioning of the two wafers is realized so as to alignthe two articulation elements 35 and 45 and form a single element 47which will be the future pivot.

Sealing can advantageously be done by the known molecular adhesiontechniques.

The two wafers A and B being assembled, the superior part of the layer41 of the B wafer is separated from the A and B assembly at the level ofthe embrittlement zone 46. This separation can advantageously be doneusing a thermal and/or mechanical treatment. After this separation,there remains only (see FIG. 5 b) a thin layer of monocrystallinesilicon 41′ eventually comprising zones of different dopings.

If the layer 41′ is too thin, the method can in addition comprise (seeFIG. 5 c) an epitaxy step for increasing the thickness of themonocrystalline film 41′ in order to increase the mechanical rigidity ofsame which will form the moveable part of the mirrors; this step may befollowed by a mechanochemical polishing step for planarizing thesurface. The final thickness of this layer 41′ is, for example, 5 to 60μm.

A layer 48 of high reflectivity of the working optical wavelengthseither metallic or dielectric multilayer is then deposited on the layer41′.

FIG. 5 d represents the following etching step of the layers 41′ and 48according to the desired pattern for the mobile part of the futuremicro-mirror. This etching is done over a mask (not shown).

FIG. 5 e represents the step of clearing the moveable part around thepivot 47 by suppression of the sacrificial layers of thermal silica bychemical attack as described for FIG. 2 h, for example.

The different manufacturing steps presenting in the various FIGS. 2 to 5can comprise numerous alternatives. In particular, the order of thedifferent steps can, in certain cases, be reversed and certain of thesteps can be modified.

Thus, for example, a single thermal oxidation layer could be realized onthe A wafer and thus form the pivot using a single element in thislayer; the monocrystalline silicon layer would be transferred directlyonto this oxide layer.

Likewise, in lieu of creating a pivot, two articulation elements (in onepart or in two parts, respectively) could be created in the thermaloxide in such a fashion as to form a hinge; in this instance, thearticulation elements are preferably disposed on either side of themoveable part and between it and the fixed part.

The moveable part could also have been realized in two parts as in theprior art and an intermediate hinge formed by etching using appropriatepatterns of the monocrystalline silicon layer.

In order to simplify the description, the connection lines of theelectrodes and the contacts to the control electronics are notrepresented in the previous figures.

These connection lines can be realized in different ways and inparticular by ionic implantation of dopants, whether or not followed bythermal diffusion appropriate to the dopants. These lines are realizedadvantageously on the front face of the support opposite to the moveablepart, the electrode or electrodes of the moveable part being connectedto certain of these lines advantageously by means of the articulationelements. These connection lines can also be realized by plated-throughholes across the substrate, the electrode or electrodes of the moveablepart being connected to certain of these plated-through holesadvantageously by means of the articulation elements.

By way of example, FIG. 3 g only represents in dotted lines therealization across the substrate of the plated-though holes 70connecting the electrodes 33 and 33′ to contacts 71.

When the micro-mirror must turn about at least two perpendicular axes ofrotation while preserving the advantage of separating the value ofangular excursion Δθ from the dimension L of the moveable part, cavitiescompletely surrounding the pivot 47 are advantageously realized in thesubstrate. In the case, wherein the connection lines are realized on thefront face of the substrate, in order not to be cut by the cavities, theelectrical connection lines (represented by way of example in FIG. 9 anddesignated by 60) supplying the different electrodes, the substrate isetched in order form there a peripheral cavity prior to forming thedoped zones 33, 33′.

FIG. 6 represent this alternative of the method.

Using a wafer 31 (see FIG. 6 a), a cavity 36 is formed by etching doneby different methods such as reactive ionic etching (corresponding tothe shape of the cavity of FIG. 3 g), wherein the preferred chemicaletching (corresponding to the shape of the cavity of FIG. 6 b) of thecavity 36 is determined using the shape (which can be circular, square,rectangular, octagonal, etc.) and dimensions of the moveable part of themicro-mirror and the value of the total angular excursion Δθ desiredalong the different axes of rotation; the value of the total angularexcursion Δθ being otherwise capable of assuming different values Δθ₁,Δθ₂, etc. along each of the axes of rotation.

The other manufacturing steps are represented in FIG. 6 c (realizationof the doped zones), FIG. 6 d (realization of the thermal oxide), FIG. 6e (realization of a via 34 in the oxide layer), FIG. 6 f (epitaxy forrealizing the pivot part), and FIG. 6 g (planarization of the structure)can be identical to those previously described. In order to obtain thefinal structure, it is then transferred onto the wafer obtained in FIG.6 g, for example the wafer obtained FIG. 4 g and, as described withreference to FIG. 5, the rest of the steps of the method are carriedout. The micro-mirror obtained is represented in FIG. 7.

Three examples of positions of the moveable part of the pivotmicro-mirror are represented respectively in FIGS. 7 a, 7 b, 7 c.

FIG. 7 a represents the moveable part disposed in a plane parallel tothe plane of the substrate; FIG. 7 b represents the moveable part thathas pivoted on an axis of rotation perpendicular to that of the pivotand perpendicular to the plane of the figure; one of the ends of themoveable part is situated in the cavity 36; FIG. 7 c represents themoveable part that has pivoted about the same axis of rotation but at180°, the opposing end of the moveable part is situated in turn in thecavity 36.

FIG. 8 a provides a diagrammatic view in perspective of a pivotmicro-mirror 47 and FIG. 8 b diagrammatically represents a perspectiveview of a simple hinged micro-mirror 57, in this example said hingebeing realized by etching of the second layer.

As mentioned above, the advantage of the pivot micro-mirrors for certainapplications is that of making possible, in virtue of a convenientconfiguration of electrodes but without modification of the principalmanufacturing steps, swinging along several axes of rotation and inparticular along two perpendicular axes.

FIG. 9 a represents a top view of a layout of electrodes in the fixedpart. The electrodes 33 making it possible to swing the moveable partalong 2 positions about one single axis of rotation R1 are two in numberand are disposed symmetrically relative to the axis of rotation R1 thatpasses through the pivot 47; the central electrode 33′ enables, togetherwith the pivot, the electrical connection of the electrode of themoveable part.

FIG. 9 b represents an electrode geometry 33 enabling obtaining 4positions about 2 perpendicular axes of rotation R1 and R2 passingthrough the pivot; these electrodes 33 are 4 in number and are paired 2by 2, each electrode couple being disposes symmetrically relative to oneof the axes; likewise, the central electrode 33′ enables together withthe pivot the electrical connection of the electrode of the moveablepart. Thus, a large number of electrode couples 33 disposed on eitherside of an axis of symmetry can be envisaged. FIG. 9 c provides anexample of 4 axes of rotation (R1, R2, R3, R4) at 45° to each other and4 electrode couples 33 disposed in sectors around the axis of the pivot.

FIGS. 9 a, 9 b, and 9 c represent in transparency the different keyelements of the micro-mirror. The sets of bottom electrodes 33(electrodes of the fixed part) and the top electrode 43 (electrode ofthe moveable part) are represented; the bottom electrode 33′ that iselectrically connected to the top electrode by the pivot 47 is drawn indark gray while in FIG. 9 b the two sets of electrodes enabling controlof the rotation of the micro-mirror along each of the perpendicular axesof rotation are drawn using two shades of lighter but different grays.The reflecting surface 48 of the moveable part and the tracks 50 and 51of the etched zones 36 make possible the separation of the variabledimensions of the micro-mirror and total angular excursion Δθ are alsorepresented.

Also very diagrammatically represented are the connection lines 62 ofthe electrodes to the contacts 60; these contacts being capable of beingconnected to a control electronics (not shown).

The different aforementioned functionalities are can, of course, berealized in the case of utilization of a single wafer and severalwafers. However, the method utilizing at least two wafers makes possiblemore possibilities. The utilization of more than two wafers can makepossible in particular the realization of more complex structures andparticularly the realization of several superimposed moveable parts, oneover the others, by means of articulation means; at least, the lastmoveable part comprising reflector means. The superpositioning of thesemoveable parts in the planes parallel to the substrate makes it possibleto have a micro-mirror with still greater degrees of freedom. The methodof the invention is in fact applied to this type of structure, inconsidering that each moveable part is realized successively over asubstrate tat can then be either a moveable part realized prior or thefirst substrate corresponding to the fixed part.

References

-   “Mirrors on a chip”, IEEE SPECTRUM, November 1993-   L. J. Hornbeck, “Micro-machining and micro-fabrication” “95”,    October 1995, Austin (US)-   D. J. Bischop and V. A. Aksyk, “Optical MEMS answer high-speed    networking requirements”, Electronic Design, 5 Apr. 1999.

1. A method for manufacturing an optical micro-mirror including a fixedpart and a moveable part connected to the fixed part by an articulationmechanism, the moveable part including a reflector, the methodcomprising: a) forming a stack formed of a mechanical substrate, asacrificial layer of a specific thickness of thermal oxidation materialas a first layer, and an assembly for forming the moveable part andincluding at least one layer of material as a second layer; b) formingthe articulation mechanism; c) forming the moveable part by etching ofat least the second layer of material to obtain at least one pattern;and d) removing at least in part the sacrificial layer for clearing themoveable part that is then connected to a rest of the micro-mirrorcorresponding to the fixed part using the articulation mechanism,wherein: the step a) of forming the stack comprises transferring ontothe mechanical substrate the second layer, the substrate and/or thesecond layer including on their faces to be transferred a thermaloxidation layer that will form the first layer after the transferring.2. The method according to claim 1, wherein the reflector is formed onthe second layer by deposition of reflector material in monolayer ormultilayers.
 3. The method according to claim 1, wherein the sacrificiallayer comprised of thermal oxidation material has a thickness greaterthan or equal to 1 micron.
 4. The method according to claim 1, whereinthe second layer is a layer of monocrystalline material.
 5. The methodaccording to claim 1, wherein the transferring comprises sealing bymolecular adhesion.
 6. The method according to claim 1, wherein thesecond layer can be combined with an intermediate substrate using acommunication zone capable of enabling removal of the intermediatesubstrate.
 7. The method according to claim 1, wherein the articulationmechanism is formed prior to the removing d) by localized etching oflayers disposed on top of the substrate to form at least one via and byepitaxy across each via, epitaxial material in each via forming whollyor in part an articulation element of the articulation mechanism.
 8. Themethod according to claim 1, wherein the articulation mechanism isformed by: localized etchings prior to the transferring, to form atleast a first via in the layer or layers disposed on top of thesubstrate and to form at least a second via in the layer or layersdisposed on the second layer and that will be facing the substrate;epitaxy across the first via forming a first part of an articulationelement and epitaxy in the second via forming a second part of thearticulation element, the first and second parts opposing during thetransferring and forming after the transferring an articulation elementof the articulation mechanism.
 9. The method according to claim 7,wherein for forming the articulation mechanism of a micro-mirror, onesingle element of articulation is formed and disposed under the moveablepart to form a pivot for the moveable part, the pivot linking themoveable part to the fixed part.
 10. The method according to claim 1,wherein for forming the articulation mechanism of a micro-mirror, twoarticulation elements are formed and disposed on either side of themoveable part to form a hinge connecting the moveable part to the fixedpart.
 11. The method according to claim 1, wherein the articulationmechanism is formed by etching the second layer.
 12. The methodaccording to claim 1, wherein the substrate is made of silicon, thefirst layer is a thermal oxide of silicon, the second layer is comprisedof monocrystalline silicon, and the articulation mechanism is comprisedof monocrystalline silicon.
 13. The method according to claim 1, furthercomprising e) attenuating the second layer.
 14. The method according toclaim 1, further comprising e) forming in the mechanical substrate atleast one cavity facing at least one part of one of ends of the moveablepart.
 15. The method according to claim 14, wherein the at least onecavity of the substrate is formed by anisotropic etching.
 16. The methodaccording to claim 1, wherein the micro-mirror is electricallycontrolled, and further comprising e) realizing a control device byformation of facing electrodes on the substrate and on the moveablepart.
 17. The method according to claim 16, wherein when the substrateand the moveable part are at least in facing semiconductor materialsparts, the electrodes are formed by ionic implantation of dopants,whether or not followed by thermal diffusion appropriate to the dopants.18. The method according to claim 17, wherein connection lines of theelectrodes to the control device are formed by ionic implantation ofdopants followed by appropriate thermal diffusion of the dopants, theconnection lines are formed on a face of the substrate facing themoveable part, the electrode or electrodes of the moveable part beingconnected to one or plural of the connection lines by the articulationmechanism, contacts being in addition provided at ends of the connectionlines with a view of their connection to the control device.
 19. Themethod according to claim 16, wherein connection lines of the electrodesto the control device are formed by plated through holes across thesubstrate, the electrode or electrodes of the moveable part beingconnected to one or a plurality of the plated-through holes by thearticulation mechanism, contacts being in addition provided at ends ofthe connection lines with a view of their connection to the commandelectronics.
 20. The method according to claim 1, wherein the moveablepart comprises at least first and second parts, the first partcomprising the reflector and at least one second part surrounding thefirst part, the articulation mechanism connecting the second part to thefixed part and an intermediate articulation mechanism connecting thefirst part of the moveable part to the second part.
 21. The methodaccording to claim 20, wherein the intermediate articulation devicemechanism a hinge.
 22. The method according to claim 21, wherein theintermediate articulation mechanism is formed by etching of the secondlayer.
 23. The method according to claim 1, applied to formation of anarray of micro-mirrors.
 24. The micro-mirror or array of micro-mirrorsobtained according to the method defined according to claim
 1. 25. Amethod for manufacturing an optical micro-mirror including a fixed partand a moveable part connected to the fixed part by an articulationmechanism, the moveable part including a reflector, the methodcomprising: forming a stack comprising a mechanical substrate, asacrificial layer of thermal oxidation material, and an assembly layerfor forming the moveable part; forming the articulation mechanism;forming the moveable part by etching in the assembly layer to obtain atleast one pattern; and removing at least in part the sacrificial layerfor clearing the moveable part that is then connected to a rest of themicro-mirror corresponding to the fixed part using the articulationmechanism, wherein: the step of forming the stack further comprisesforming at least one thermal oxidation layer on the assembly layer,wherein the step of forming the stack further comprises transferringonto the mechanical substrate said at least one thermal oxidation layerthat will form the sacrificial layer after the transferring.
 26. Themethod for manufacturing an optical micro-mirror according to claim 25,wherein the step of forming the stack further comprises forming at leastone thermal oxidation layer on the substrate.
 27. The method formanufacturing an optical micro-mirror according to claim 25, wherein thestep of forming the stack further comprises forming at least two thermaloxidation layers.
 28. The method for manufacturing an opticalmicro-mirror according to claim 27, wherein the step of forming thestack further comprises transferring the substrate onto the assemblylayer oxidized face to oxidized face.
 29. A method for manufacturing anoptical micro-mirror including a fixed part and a moveable partconnected to the fixed part by an articulation mechanism, the moveablepart including a reflector, the method comprising: forming a stackcomprising a mechanical substrate, a sacrificial layer of a specificthickness of thermal oxidation material as a first layer, and anassembly for forming the moveable part and including at least one layerof material as a second layer; forming the articulation mechanism;forming the moveable part by etching of at least the second layer ofmaterial to obtain at least one pattern; removing at least in part thesacrificial layer for clearing the moveable part that is then connectedto a rest of the micro-mirror corresponding to the fixed part using thearticulation mechanism; and forming in the mechanical substrate at leastone cavity facing at least one part of one of ends of the moveable part.